Electric motors typically undergo mechanical and electrical degradation with use. A common occurrence with ac induction motors is rotor related failures, such as rotor bar degradation or rotor eccentricity. Motor sound, vibration, tachometers, temperature and other means have been used to detect operating motor condition.
In the case of tachometers, some tachometers require contact with the motor shaft. Others, such as photo-tachometers and stroboscope tachometers, require close proximity to the motor, and also require that a portion of the motor shaft be exposed. Many motors, however, are located in inaccessible locations or operate in hazardous environments. Personnel safety may often preclude the monitoring of certain motors even when it otherwise would be desirable. Many motors, even if accessible, do not provide an exposed shaft due to their mounting configurations. For example, many compressors used in air conditioning and refrigeration equipment are coupled to their motors inside a sealed compartment thus preventing motor speed measurements by all commercially available tachometers.
Recent improvements in the diagnostics of electric motors and loads have included the development of motor current signature analysis systems. These systems usually employ a spectrum analyzer, and may obtain the motor current from a current probe clamped over the electrical leads to the motor.
An important advantage of motor current systems is that they permit the motor and load sensing to be done remotely, even at some distance from the actual motor under test. All that is needed is access to the motor leads. Oak Ridge National Laboratory patents such as U.S. Pat. No. 4,965,513 (to Haynes and Eissenberg) and U.S. Pat. No. 4,978,909 (to Hendrix and Smith) pioneered the field of motor current signature analysis.
Commercially available motor diagnostic systems include MotorCheck.TM. from Computational Systems Incorporated (CSI) and MotorMonitor.TM. from Entek Scientific Corporation. Both MotorCheck.TM. and MotorMonitor.TM. are computer-based motor current monitoring systems that diagnose the condition of an ac-powered electric induction motor by examining the slip-pole sidebands, or "fault frequencies" in the raw motor current frequency spectrum. While such systems can effectively identify degraded motors, their cost and complexity are disadvantages to their widespread use. The commercial systems are generally expensive (costing &gt;$10,000), and require a trained operator.
The subject invention differs from the above commercial systems in several material respects. First, the commercial systems monitor sideband amplitudes, typically sidebands of the 60 Hz line frequency. The subject invention does not use sideband measurements at all, but relies instead on demodulating the ac current to isolate a slip-poles component which becomes the basis for all the subsequent circuits of the invention. A second difference is that the invention is intended for quick on-the-spot motor diagnoses by relatively untrained observers. The commercial systems, on the other hand, are best suited for use by highly trained engineers or technicians whose objective is to obtain detailed motor analyses. Thirdly, the subject invention provides a remote tachometer, which is also based on the slip-poles component. Additionally, the commercial systems are computer-based fast fourier transform analysis systems, whereas the subject invention is an analog-based hand-held instrument.
The instrument, in other words, has as one object to be capable of use by relatively unskilled observers to quickly identify those motors that may be in the process of failing. Later, one of the more expensive motor current diagnostic systems could be used to more accurately assess the condition of any motors that were found to be suspect.
Another object is to provide a small, inexpensive, simple-to-use, and easy-to-understand remote motor monitor and tachometer.
Another object is to provide simple indications of motor speed, motor running current, motor condition (e.g., rotor bar condition) and an audible indication of motor condition based on the magnitude of the conditioned slip-poles component. This can be packaged in one small instrument without the need for expensive computer-based or spectrum-analyzer-based instrumentation.
Still another object of the invention is to enable a relatively untrained worker to diagnose an electric motor in the field without requiring the presence of a trained engineer or technician.